For decades, many studies have been carried out in order to develop and optimize air electrodes that make it possible to produce electrochemical generators of metal-air type, which are known for their high energies by weight, that can reach several hundred Wh/kg.
Air electrodes are also used in alkaline fuel cells which are particularly advantageous compared with other systems owing to the high reaction kinetics at the electrodes.
An air electrode makes it possible to use, as oxidizing agent for the electrochemical reaction, air from the atmosphere, which is available in unlimited amount anywhere and at any time.
An air electrode is a porous solid structure in contact with the liquid electrolyte, which is generally an alkaline solution. The interface between the air electrode and the liquid electrolyte is a “triple contact” interface where the active solid material of the electrode, the oxidizing gas (air) and the liquid electrolyte are simultaneously present. This triple contact interface has always posed numerous difficulties linked in particular to the gradual degradation, even when not operating, of the air electrode, in particular when the liquid electrolyte is a concentrated alkaline solution, such as a several times molar solution of sodium hydroxide, potassium hydroxide or lithium hydroxide.
The drawbacks of air electrodes in alkaline fuel cells are set out, for example, in the literature article by G. F. McLean et al., entitled “An assessment of alkaline fuel cell technology”, International Journal of Hydrogen Energy 27 (2002), 507-526:                these electrodes undergo gradual wetting of their porous structures until flooding thereof which makes them ineffective. This change is accelerated during operation of the fuel cell or of the battery;        in the long term, the carbon dioxide present in the air diffuses toward and dissolves in the alkaline solution forming the electrolyte, in the form of a carbonate anion which precipitates in the presence of alkaline cations (Na, K, Li). A gradual and inevitable carbonation of the electrolyte is thus observed;        carbonates form mainly at the liquid/porous solid interface and promote the flooding phenomenon mentioned above;        the carbonate precipitation gradually destroys the structure of the air electrode and considerably reduces the charge transfer kinetics at the triple point, which ends up making the electrode ineffective.        
The objective of the present invention was to develop anion-conducting cationic polymer materials capable of being interposed between the air electrode and the liquid alkaline electrolyte or else of replacing the latter, in order to substantially reduce, or even eliminate, the carbonation of the solid electrolyte and the degradation of the air electrode which results therefrom.
Such materials should be usable in alkaline fuel cells and metal-air batteries, either as a solid electrolyte, or as membrane separating the air electrode from the alkaline liquid electrolyte.
The applicant has proposed, in international application WO 2006/016068, a crosslinked organic polymer material, in particular as anion-conducting solid electrolyte in alkaline fuel cells. The polymer material described in this application is obtained by a nucleophilic substitution between a halogenated linear polymer, such as polyepichlorohydrin, and a combination of at least one tertiary diamine and of at least one tertiary or secondary monoamine. The reaction between the tertiary amine functions and the chlorinated functions of the polymer results in the formation of quaternary ammonium functions responsible for the anion-conducting power of the polymer material obtained. Moreover, the reaction of the two tertiary amine functions of the bifunctional reactant (tertiary diamine) results in the crosslinking of the polymer and in the formation of an insoluble three-dimensional network.
However, this crosslinked anion-conducting material based on polyepichlorohydrin comprising quaternary ammonium groups is not stable in concentrated alkaline solutions. Moreover, it is not self-supported and, in order to obtain it in the form of a large membrane that can be handled, it is necessary to synthesize it on a support or in a porous or fibrous structure, for example a nonwoven textile made of polypropylene.
The applicant, in the context of its research aimed at developing improved anion-conducting organic materials capable of being used in fuel cells or batteries in order to reduce the degradation of air electrodes, has discovered that it is possible to overcome the drawbacks described above, by incorporating a polymeric system as described in WO 2006/016068 into an interpenetrating polymer network (IPN) or a semi-interpenetrating polymer network (semi-IPN).
An interpenetrating polymer network (IPN) is a polymeric system comprising at least two networks of polymers of which at least one has been synthesized in the presence of the other, without, however, being linked to one another by covalent bonds, and which cannot be separated from one another without breaking chemical bonds (IUPAC Compendium of Chemical Terminology, 2nd edition, 1997).
A semi-interpenetrating polymer network (semi-IPN) differs from an IPN by virtue of the fact that one of the at least two polymers present does not form a three-dimensional network, i.e., is not crosslinked, but is a linear or branched polymer. Owing to the absence of crosslinking of the second polymeric system, the latter can be separated from the first by extraction.